1. Field of the Invention
The present invention relates to a solid-state image-sensing device in which variations in sensitivity among individual pixels are corrected for.
2. Description of the Prior Art
In a solid-state image-sensing device having photosensitive elements such as photodiodes, from one pixel after another, an image signal obtained as a result of image sensing and a noise signal representing the variation in sensitivity of that particular pixel are output by way of an output signal line. Here, by subtracting the noise signal of each pixel from the image signal of that pixel, it is possible to correct for variations in sensitivity among individual pixels. In a conventional solid-state image-sensing device, the image signal and the noise signal are fed through separate buffers to the non-inverting input terminal and the inverting input terminal, respectively, of a differential amplifier circuit, which then outputs the image signal corrected for the variation in sensitivity.
Feeding the image and noise signals through separate buffers in this way, however, results in the image signals from the pixels in one column being corrected to a different degree from the image signals from the pixels in another column, due to variations in the characteristics of the circuit elements constituting the two buffers. That is, when the image signals, fed through a plurality of buffers having different amplification factors, are corrected by the use of their respective noise signals, fed through a plurality of buffers having different amplification factors, the image signals are corrected to different degrees. These variations in the degree of correction appear as vertical stripes in the image reproduced from the image signals output from the differential amplifier circuit.
As an attempt to prevent fixed-pattern noise like the vertical stripes described above, the applicant of the present invention has proposed a solid-state image-sensing device in U.S. Patent Application Serial No. 2001/0013571 A1. In the solid-state image-sensing device proposed in this publication, each column of pixels is provided with a selection circuit 100 configured as shown in FIG. 11. A current signal output from a pixel G to an output signal line 101 is converted into a voltage signal by a constant-current source 102, and this voltage signal is fed to the selection circuit 100. When the signal thus fed in is an image signal, a switch SWa is turned on so that the image signal is sampled and held in a capacitor Ca; when the signal fed in is a noise signal, a switch SWb is turned on so that the noise signal is sample and held in a capacitor Cb.
As shown in FIG. 11, the output from the selection circuit 100 provided in each column is fed, by way of a single signal transmission line, to a correction circuit 103, of which only one is provided in one solid-state image-sensing device. At this time, in the selection circuit 100, a switch SWr is turned on to reset the input side of a buffer Ba, and then a switch SW1a is turned on so that the image signal sampled and held in the capacitor Ca is output through the buffer Ba. At this time, in the correction circuit 103, a switch SW2a is turned on so that the image signal is sampled and held in a capacitor Cc.
Then, in the selection circuit 100, the switch SWr is turned on to reset the input side of the buffer Ba, and then a switch SW1b is turned on so that the noise signal sampled and held in the capacitor Cb is output through the buffer Ba. At this time, in the correction circuit 103, a switch SW2b is turned on so that the noise signal is sampled and held in a capacitor Cd.
When the image and noise signals are sampled and held in the correction circuit 103 in this way, they are fed through buffers Bb and Bc to the non-inverting input terminal and the inverting input terminal, respectively, of a differential amplifier circuit 104, which then outputs the image signal cleared of the noise component. Thus, the image and noise signals from the pixels belonging to the same column are fed, all through the same buffer Ba provided in that column, to the correction circuit 103. As a result, the image signals output from the correction circuit 103 are corrected for variations due to the characteristics of the buffer Ba. This helps alleviate fixed-pattern noise.
In the solid-state image-sensing device proposed in United States Patent Application Serial No. 2001/0013571 A1, the image and noise signals are output to the correction circuit 103 with different timing. Therefore, for this solid-state image-sensing device to output signals at a rate comparable with that of a solid-state image-sensing device that outputs image signals alone without clearing them of noise components, the selection circuit 100, which outputs both image and noise signals to the correction circuit 103, needs to operate at twice the rate at which image signals are read out.
However, there is a limit to the rate at which image signals can be read out. Specifically, if image signals are read out too fast, it is not possible to secure a sufficient exposure time, and therefore it is not possible to obtain sufficiently high outputs. Thus, the selection circuit 100 needs to operate at a rate reduced according to the rate at which image signals are read out. As a result, the operation of the solid-state image-sensing device as a whole takes an accordingly long time, imposing a limit on the highest possible rate at which it can operate.